The principle of wide-imaging stereo which is used in a number of loudspeakers being manufactured by the present applicants is disclosed in Patent GB-B-2188811. That patent discloses an acoustic reflector provided opposite to a diaphragm of a drive unit which determines the directivity of the speaker output. The reflector is conical and is located with its axis offset from the axis or centre line of the drive unit so as to produce an asymmetric sound distribution, with most of the mid range and high frequency sound being emitted towards an intended listening direction (hereinafter "forwardly"), but in a wide rather than a narrow distribution.
The use of a pair of right and left speakers and a pair of audio mirrors for respectively controlling the directivities of sounds which are output from the pair of speakers, with the shapes or arrangement of the pair of mirrors being adjusted so that a difference between the arrival times of the sounds which are respectively output from the pair of speakers can be compensated by a sound pressure difference due to the Hass effect in a predetermined area is disclosed in our EP-A-0320270. That specification also discloses that the sound mirror should have concave sides when viewed in profile so as to widen the sound distribution vertically as well as in a horizontal plane.
Our patent specification WO 92/07449 discloses a speaker unit having a drive unit fitting within a generally hemispherical housing which is supported cantilever-wise above a generally conical sound mirror. The axis of the drive unit is offset relative to the axis of the sound mirror to enhance the distribution of high frequency and mid range sound forwardly towards a preferred listening direction. The drive unit has concentric first and second diaphragms provided in one embodiment by a parasitic tweeter, and directs a narrow beam of high frequency and mid range sound towards the sound mirror, from which it can be reflected into a room to produce the desired sound distribution. The use of a single drive unit with concentric diaphragms for high frequency and mid range (and optionally also low frequency) sound gives rise to a relatively easily controllable pattern of reflected and diffracted sound. The housing is supported above the sound mirror on a pair of struts which have lengths greater than their widths and are directed towards the axis of the drive unit so as to minimise their effect on the sound reaching the listener. In the practical form of that speaker, the housing and the sound mirror are separate components between which there is no gas flow path, so that the only gas to provide a reflex load for the drive unit is that in the housing behind the drive unit. The need for a given gas volume to provide an adequate load for the drive unit imposes a minimum size on the enclosure within which that drive unit is contained. A further feature of the practical form of the speaker disclosed in this specification is that the case for the speaker, including the sound mirror, is made of metal which adds to the cost. Furthermore, if one drive unit is changed for another having different dispersion characteristics, it may be desirable to redesign the mirror to take account the different characteristics, and this cannot easily be done without retooling, which imposes a practical limit on the range of different drive units that can be used.
One problem with which the invention is concerned is to provide a compact loudspeaker casing for a wide dispersion or wide-imaging stereo loudspeaker which contains a relatively large effective internal volume that provides adequate reflex loading for the drive unit to achieve a desired low-frequency response.
A further problem with which the invention is concerned is the provision of a loudspeaker unit in which the casing, including the audio mirror, can be made largely or completely of plastics without introducing unacceptable resonances resulting from the limited rigidity of the plastics material.
A yet further problem with which the invention is concerned is the provision of a case for a loudspeaker to be used in wide-imaging stereophonic reproduction of sound, in which there can be fitted a range of drive units of different dispersion patterns without redesign of the case as a whole.